/*
 * Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *   - Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *
 *   - Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *
 *   - Neither the name of Oracle nor the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
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 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

package utils;

import static java.nio.file.LinkOption.NOFOLLOW_LINKS;
import static java.nio.file.StandardWatchEventKinds.ENTRY_CREATE;
import static java.nio.file.StandardWatchEventKinds.ENTRY_DELETE;
import static java.nio.file.StandardWatchEventKinds.ENTRY_MODIFY;
import static java.nio.file.StandardWatchEventKinds.OVERFLOW;

import java.io.IOException;
import java.nio.file.ClosedWatchServiceException;
import java.nio.file.FileSystems;
import java.nio.file.FileVisitResult;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.SimpleFileVisitor;
import java.nio.file.WatchEvent;
import java.nio.file.WatchEvent.Kind;
import java.nio.file.WatchKey;
import java.nio.file.WatchService;
import java.nio.file.attribute.BasicFileAttributes;
import java.util.HashMap;
import java.util.Map;
import java.util.function.BiFunction;

/**
 * Utility class for watching a directory (or tree) for changes to files.
 * 
 * @see <a
 *      href="http://docs.oracle.com/javase/tutorial/essential/io/examples/WatchDir.java">WatchDir.java</a>
 *      (the example this class is based on).
 */
public class DirectoryWatcher {
	private final WatchService watcher;
	private final Map<WatchKey, Path> keys;
	private final boolean recursive;
	private BiFunction<Kind<?>, Path, Void> callback;
	private boolean trace = false;

	/**
	 * Creates a WatchService and registers the given directory.
	 * 
	 * @param dir the Path to the directory.
	 * @param recursive true to recursively watch the contents of {@code dir}.
	 * @param callback the function to call when a watched file is changed.
	 * @throws IOException if a file error occurs.
	 */
	public DirectoryWatcher(Path dir, boolean recursive, BiFunction<Kind<?>, Path, Void> callback) throws IOException {
		this.watcher = FileSystems.getDefault().newWatchService();
		this.keys = new HashMap<WatchKey, Path>();
		this.recursive = recursive;
		this.callback = callback;
		if (recursive) {
			// System.out.format("Scanning %s ...\n", dir);
			registerAll(dir);
			// System.out.println("Done.");
		} else {
			register(dir);
		}
		// Enable trace after initial registration
		this.trace = true;
	}

	@SuppressWarnings("unchecked")
	private static <T> WatchEvent<T> cast(WatchEvent<?> event) {
		return (WatchEvent<T>) event;
	}

	/**
	 * Registers the given directory with the WatchService.
	 * 
	 * @param dir the Path to the directory.
	 * @throws IOException if a file error occurs.
	 */
	private void register(Path dir) throws IOException {
		WatchKey key = dir.register(this.watcher, ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY);
		if (this.trace) {
			Path prev = this.keys.get(key);
			if (prev == null) {
				// System.out.format("register: %s\n", dir);
			} else {
				if (!dir.equals(prev)) {
					// System.out.format("update: %s -> %s\n", prev, dir);
				}
			}
		}
		this.keys.put(key, dir);
	}

	/**
	 * Registers the given directory, and all its sub-directories, with the
	 * WatchService.
	 * 
	 * @param start the Path to the directory.
	 * @throws IOException if a file error occurs.
	 */
	private void registerAll(Path start) throws IOException {
		// Register directory and sub-directories
		Files.walkFileTree(start, new SimpleFileVisitor<Path>() {
			@Override
			public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
				register(dir);
				return FileVisitResult.CONTINUE;
			}

			@Override
			public FileVisitResult visitFileFailed(Path file, IOException e) throws IOException {
				System.err.printf("Visiting failed for %s\n", file);
				return FileVisitResult.SKIP_SUBTREE;
			}
		});
	}

	/**
	 * Closes the WatchService.
	 */
	public void unregister() {
		try {
			this.watcher.close();
			// System.out.println("Watcher closed.");
		} catch (IOException e) {
			e.printStackTrace();
		}
	}

	/**
	 * Processes all events for keys queued to the watcher.
	 */
	public void processEvents() {
		for (;;) {
			// Wait for key to be signaled
			WatchKey key;
			try {
				key = this.watcher.take();
			} catch (InterruptedException | ClosedWatchServiceException x) {
				return;
			}
			Path dir = this.keys.get(key);
			if (dir == null) {
				System.err.println("WatchKey not recognized!!");
				continue;
			}
			for (WatchEvent<?> event : key.pollEvents()) {
				Kind<?> kind = event.kind();
				// TBD - provide example of how OVERFLOW event is handled
				if (kind == OVERFLOW) {
					continue;
				}
				// Context for directory entry event is the file name of entry
				WatchEvent<Path> ev = cast(event);
				Path name = ev.context();
				Path child = dir.resolve(name);
				// Print out event
				// System.out.format("%s: %s\n", event.kind().name(), child);
				this.callback.apply(event.kind(), child);
				// If directory is created, and watching recursively, then
				// Register it and its sub-directories
				if (this.recursive && (kind == ENTRY_CREATE)) {
					try {
						if (Files.isDirectory(child, NOFOLLOW_LINKS)) {
							registerAll(child);
						}
					} catch (IOException x) {
						// Ignore to keep sample readable
					}
				}
			}
			// Reset key and remove from set if directory no longer accessible
			boolean valid = key.reset();
			if (!valid) {
				this.keys.remove(key);
				// All directories are inaccessible
				if (this.keys.isEmpty()) {
					break;
				}
			}
		}
	}
}
